In recent years, in the fields, such as vehicles, security robots, unmanned helicopters, and the like, demands to perform obstacle detection in a proceeding direction accurately for the purposes of collision prevention, have been increasing. As methods of detecting obstacles, laser radars which are distance measuring apparatuses utilizing optical scanning, have been known. General laser radars are configured to rotate or oscillate a mirror, a polygon mirror or the like while projecting light fluxes emitted from a laser light source onto the mirror, the polygon mirror, or the like so as to scan a wide range and to receive scattered light rays from a light-projected object with a light receiving element, thereby performing shape measurement and distance measurement.
PTL 1 discloses a technique with regard to a polygon mirror which includes the even number of planar reflective surfaces and performs scanning by reflecting light rays the even number of times.